BMI, Body Fat or Calories: Which Number Should You Track?

Body mass index is weight in kilograms divided by height in metres squared. The World Health Organization’s adult bands are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight and 30 or more obesity. For a 35-year-old man weighing 190 lb at 5 ft 10 in, the BMI calculator gives 27.3 — overweight — with a healthy-range weight for that height of 128.9 lb – 173.5 lb.

BMI is quick and useful for populations, but it cannot distinguish muscle from fat or show where fat is stored. A muscular athlete and a sedentary person can share the same BMI.

Body fat percentage separates fat from lean mass. The US Navy method estimates it from height, neck and waist (and hips for women). For the same man with a 15.5 in neck, a 36 in waist gives 20.2% and a 34 in waist gives 16.5%. Two inches off the waist moved the estimate by about two percentage points — which shows both how sensitive the method is to measurement and why you should measure the same way each time.

The typical error against laboratory methods is around 3–4 percentage points, so use it for trends. The American Council on Exercise publishes category ranges such as about 14–17% for “fitness” and 18–24% for “average” in men.

Total daily energy expenditure combines your basal metabolic rate with an activity multiplier. For the same man, the Mifflin–St Jeor equation estimates a BMR of 1,803 kcal, and the result changes a lot with how active you say you are:

The activity choice is the biggest source of error, because most people overrate it. Pick the lower option if unsure, then adjust after two to three weeks of real data.

• High BMI, low body fat: typical of muscular, strength-trained people. BMI overstates risk.
• Normal BMI, high body fat: sometimes called “skinny fat”; BMI understates risk.
• TDEE vs reality: equations predict averages. Individuals vary by 10% or more, so the calorie number is a hypothesis to test.

1. Use BMI as a first screen and a conversation starter with a clinician.
2. Use body fat or waist measurements, taken consistently, to track composition over months.
3. Use TDEE to set a calorie estimate, then compare it with your weight trend and adjust by 100–150 kcal at a time.
4. Prioritise what you can feel and do: strength, stamina, sleep, diet quality and blood tests matter more than any single number.

If you are pregnant, under 18, managing a medical condition or worried about eating, talk to a doctor or registered dietitian rather than relying on calculators.

A practical routine takes five minutes a week. Weigh yourself on the same mornings under the same conditions, measure your waist once a week at the same point, and note how you feel and perform in training. After three weeks compare the trend with your calorie estimate. If your weight is flat while you intended to lose, trim 100–150 kcal a day or add a little activity; if it falls faster than planned, eat a bit more. Small, patient adjustments beat dramatic changes, and they protect muscle, energy and mood. Keep records in a simple notebook or spreadsheet so you are reacting to trends, not to a single high or low reading caused by water, salt or timing.